A tandem structure solar cell device comprising a GaAs solar cell serially disposed on a Si solar cell utilizes solar light more effectively than either a Si solar cell or a GaAs solar cell alone.
FIG. 2 is a cross-sectional view of a prior art tandem solar cell device. In FIG. 2, a Si solar cell (lower solar cell) 10 includes an n-type Si substrate 11 about 100 to 200 microns thick and a p-type Si layer 12 less than 1 micron thick disposed thereon. Light having wavelengths of 0.4 to 1.1 microns is absorbed by and converted to electricity by the Si solar cell 10. A GaAs solar cell (upper solar cell) 20 includes an n-type GaAs layer 21 about 2 microns thick and a p-type GaAs layer 22 about 1 micron thick disposed thereon. Light having wavelengths of 0.4 to 0.9 microns is absorbed by and converted to electricity by the GaAs solar cell 20. A buffer layer 30 less than several hundred angstroms thick is inserted between the upper solar cell 20 and the lower solar cell 10, improving the lattice matching of the Si and GaAs crystals. This buffer layer 30 includes a tunnel junction 33 and a high impurity concentration p.sup.+ -type Ge layer 31 and an n.sup.+ -type Ge layer 32 sandwiching the tunnel junction.
An n side ohmic contact electrode 1 is disposed on the rear surface of the n-type Si substrate 11. A p side ohmic contact electrode 2 is disposed on part of the front surface of the p-type GaAs layer 22. An anti-reflection film 40 comprising a silicon nitride film having a thickness of 600 to 700 angstroms is disposed on the exposed surface of GaAs layer 22.
Thus, a tandem solar cell device 100 includes the upper solar cell 20, the lower solar cell 10, the buffer layer 30, the anti-reflection film 40, the p side electrode 2, and the n side electrode 1.
In the tandem solar cell device described, light of relatively short wavelengths, i.e., of 0.4 to 0.9 microns, from the solar light spectrum of 0.4 to 2 micron wavelengths which is incident on the device from above is converted into electricity by the upper GaAs solar cell 20, and the light that passes through the upper solar cell 20 is converted into electricity by the lower Si solar cell 10. The charge carriers generated at the respective solar cells 10 and 20 are extracted through the electrodes 1 and 2 as a photocurrent that passes through the thin buffer layer 30 disposed between the two solar cells.
In the prior art tandem solar cell device, the lattice matching buffer layer 30 cuts off light which could be converted to electricity by the lower solar cell 10 and little electricity is produced in the lower solar cell 10.
In more detail, when a plurality of semiconductor layers having different lattice constants are serially disposed, crystalline defects are produced and a semiconductor layer of good crystallinity cannot be obtained. The movement of charge carriers in the semiconductor layer is obstructed by the crystalline defects, thereby decreasing device efficiency. Therefore, in this prior art device, the Ge buffer layer 30 whose crystal lattice constant matches that of GaAs is inserted between the p-type Si layer 12 and the GaAs layer 21, thereby improving the crystallinity of the GaAs. Furthermore, in order to improve the electrical junction between the upper GaAs solar cell 20 and the lower Si solar cell 10, the buffer layer 30 includes a p.sup.+ -type Ge layer 31 and an n.sup.+ -type Ge layer 32 containing a high concentration of dopant impurities and a tunnel junction 30a is produced within the buffer layer 30.
The energy band gap of the Ge buffer layer 30 is narrower than that of Si, i.e., the band gap energy of Si is 1.11 eV while that of Ge is 0.66 eV. As a result, light that can be converted into electricity by the Si layer is absorbed by the Ge buffer layer 30 and hardly reaches the Si solar cell 10; that is, the light-to-electricity conversion takes place not in the Si layer having a high light-to-electricity conversion efficiency but in the Ge layer having a low light-to-electricity conversion efficiency. This lowers the light-to-electricity conversion efficiency of the entire device.